1. Field of the Invention
The present invention relates to an optical amplifier and optical amplification method for amplifying wavelength division multiplexed (WDM) light which includes light in different wavelength bands.
2. Description of the Related Art
Research and development in the area of wavelength division multiplexed (WDM) optical communication systems has resulted in a steady increase in the number of wavelengths being multiplexed together. In addition, the wavelength bands for transmission are being widened.
Furthermore, research and development is also advancing the development of WDM optical communication systems which utilize optical amplifiers as linear repeaters. With such WDM optical communication systems, a plurality of signal lights in a wavelength band of, for example, 1.53 to 1.56 μm (hereinafter referred to as a 1.55 μm band), can be collectively amplified with an optical amplifier, thereby enabling large-capacity and long-distance light transmission with a simple construction.
In addition, optical communication systems which address band expansion of an optical amplifier have also been proposed. For example, optical amplifiers which can amplify signal lights in a long wavelength band of, for example, 1.57 to 1.60 μm (hereinafter referred to as a 1.58 μm band) have been proposed.
For example, FIG. 1 is a diagram showing a conventional optical amplifier for amplifying WDM signal light which includes both signal light in a 1.55 μm band (1.53 to 1.56 μm) and signal light in a 1.58 μm band (1.57 to 1.60 μm). With typical optical amplifiers, in particular optical fiber amplifiers, it is difficult to obtain an equal gain over a wide band exceeding 60 nm. Therefore, the optical amplifier in FIG. 1 divides the WDM signal light into, for example, two bands of 1.55 μm and 1.58 μm, and obtains equal gains over the respective bands.
Referring now to FIG. 1, WDM signal light from a single optical fiber is demultiplexed by a WDM coupler 1 into WDM signal lights of a 1.55 μm band and a 1.58 μm band. Then, the WDM signal lights of the 1.55 μm band and the 1.58 μm band are directed to a 1.55 μm band optical fiber amplifier section 2 and a 1.58 μm band optical fiber amplifier section 3, respectively. The respective WDM signal lights amplified by optical fiber amplifier sections 2 and 3 are then multiplexed in a WDM coupler 4, and output to a single optical fiber.
However, various problems can occur with an optical communication system which transmits signal light over a wide wavelength band. For example, assume signal light of the 1.55 μm band is transmitted over a long distance using an optical transmission path comprising, for example, a single mode optical fiber (SMF) which transmits the wavelength close to 1.3 μm with zero dispersion. In this case, there is a problem that the transmitted waveform becomes distorted if the signal light is transmitted at a high transmission speed. This distortion is due to the wavelength dispersion characteristics of the optical transmission path.
For example, with a general 1.3 μm zero dispersion SMF, there is a wavelength dispersion of approximately 18 ps/nm/km in the vicinity of 1.55 μm. For example, in the case where a signal light of 1.55 μm is transmitted 50 km, then a wavelength dispersion of 900 ps/nm (=18 ps/nm/km×50 km) accumulates. This is generally referred to as primary dispersion, and indicates that a delay difference of 900 ps per wavelength amplitude of 1 nm is produced.
Whether or not this delay difference exerts an influence on the transmission characteristics is related to the time slot of the signal light. That is to say, in the case where the time slot of the signal light is sufficiently longer than the delay difference due to the wavelength dispersion, the influence on the transmission waveform is minimal. However, when the time slot approaches the delay difference, the influence of the wavelength dispersion increases so that the waveform becomes distorted. In general, it is considered that if the transmission speed of the signal light per unit wavelength exceeds approximately 2.5 Gb/s, then compensation for wavelength dispersion is required. For example, in the case where the transmission speed of the signal light is 10 Gb/s, the time slot becomes 100 ps, and the wavelength dispersion of 900 ps/nm for the above mentioned case exerts a considerable influence on the transmission characteristics.
To compensate for the wavelength dispersion characteristics of the optical fiber transmission path, the light signal may be passed through a dispersion compensator having opposite wavelength dispersion characteristics to the transmission path. In the case of compensating for a wavelength dispersion of 900 ps/nm, a dispersion compensator having a wavelength dispersion of −900 ps/nm is used. For example, a dispersion compensating fiber (DCF) is widely used as such a dispersion compensator.
However, in the case where compensation is performed with a wavelength dispersion of 1.55 μm as a reference, as the wavelength band of the signal light is increased, the compensation error increases as the deviation of the wavelength from 1.55 μm increases.
For example, FIG. 2 is a diagram showing wavelength dispersion characteristics for a 1.3 μm zero dispersion SMF. As shown in FIG. 2, the wavelength dispersion characteristic of a 1.3 μm zero dispersion SMF has an incline with respect to wavelength. As a result, for example, a wavelength dispersion with respect to a signal light of 1.53 μm becomes 18−Δs ps/nm/km, and a wavelength dispersion with respect to a signal light of 1.58 μm becomes 18+ΔL ps/nm/km. Consequently in the case where 50 km transmission is performed, then even if a dispersion compensator having a compensation amount of the abovementioned −900 ps/nm is used, the Δs×50 ps/nm component is excessively compensated for with respect to the signal light of 1.53 μm, while the ΔL×50 ps/nm component is insufficiently compensated for with respect to the signal light of 1.58 μm. The wavelength dispersion produced due to this situation where the wavelength dispersion characteristics of the optical fiber transmission path have an incline with respect to the wavelength is generally referred to as secondary dispersion, and when the number of wavelengths of the signal light is large and the wavelength band is wide, it is necessary to perform compensation not only for primary dispersion but also for secondary dispersion.
As mentioned above, a high speed WDM optical communication system with a transmission speed per unit wavelength exceeding 2.5 Gb/s, using a 1.55 μm band or a 1.58 μm band as the band for wavelength division multiplexed signal light is, currently being developed. In realizing such a system, an important consideration is how to compensate for the primary and secondary wavelength dispersion to improve efficiency. Furthermore, it is considered that when wavelength dispersion compensation in the above mentioned wide wavelength band is collectively performed, a signal light of large power is transmitted to the dispersion compensator. Therefore, for example, a nonlinear optical effect such as cross-phase modulation (XPM) or four-wave mixing (FWM) is likely to occur, so that there is the likelihood of degradation of the transmission characteristics.